In a conventional manufacturing method of a grain-oriented electrical steel sheet, an insulating film called as a glass coating film is formed at a surface of a steel strip at finish annealing, and a control of a crystal orientation using AlN precipitates as an inhibitor is performed. A tensile tension acts on the steel strip by the glass coating film, and thereby, a core loss of the grain-oriented electrical steel sheet is reduced. There is a case when the glass coating film is called as a forsterite film or a primary coating film. Besides, excitation properties improve owing to the control of the crystal orientation.
However, there is a case when a number of defects occur at the glass coating film in the conventional manufacturing method as stated above. A size of the defect in a direction parallel to the surface of the steel strip is several dozen μm to several hundred μm. If the defect as stated above occurs, an external appearance deteriorates because the steel strip exposes from the glass coating film. Besides, the defect of the glass coating film leads to the core loss and/or deterioration of the excitation properties.
A study to reduce the defects of the glass coating film has been made, but it is impossible to fully reduce the defects by the former technologies.